The present invention relates to an apparatus for retaining assemblies, such as a tubing hanger, in a wellhead assembly and to a method for using the same. The present invention relates particularly to the apparatus in the form of an internal tree cap for use in a spool tree.
A wellhead assembly, such as that employed on the seabed for offshore drilling and production operations, will often comprise an apparatus for retaining equipment and assemblies within the wellhead. An example of such a device is an internal tree cap. Internal tree caps typically comprise a housing for location within the bore of the wellhead assembly. The internal tree cap further comprises an assembly for engaging a suitable means within the wellhead. Suitable means within the bore of the wellhead to be engaged by the internal tree cap include one or more circumferential internal grooves and one or more internal seats or shoulders. Internal tree caps are employed within a wellhead assembly to retain assemblies and equipment. For example, internal tree caps can be employed to retain one or more plugs, such as a wireline plug, to close the central bore of the wellhead assembly. One or more seals are typically disposed between the tree cap and the inner wall of the wellhead assembly. One common location for internal tree caps is within the central bore of a spool tree forming part of the wellhead assembly.
In a further application, an internal tree cap is employed to retain a casing or tubing hanger within the wellhead. Within a well, casings or tubings are installed suspended from a hanger. The hanger is typically landed within the bore of the wellhead assembly against a shoulder. The hanger assembly typically comprises one or more seals to seal against the inner wall of the wellhead assembly. The hanger assembly may comprise one or more assemblies for engaging with the wellhead, for example with one or more circumferential grooves formed in the inner wall of the wellhead assembly.
It has been found that, in use, movement of such devices as internal tree caps and tubing hangers within the bore of the wellhead assembly after installation causes rapid wear of the seals between the respective devices and the inner wall of the wellhead assembly to occur. This in turn leads to a loss in the integrity of the seal.
U.S. Pat. No. 4,408,783 discloses a holddown apparatus for retaining a casing or tubing hanger within a wellhead assembly. The holddown apparatus is mounted on the tubing hanger. The hanger is provided with a shoulder for engaging with a seat in the bore of the wellhead assembly. The holddown assembly comprises a radially expansible locking ring having an upwardly facing tapered surface. An annular cam ring is provided having a corresponding tapered surface to engage with the tapered surface of the locking ring and is threadingly engaged with the hanger. When the hanger and holddown apparatus are located within the wellhead assembly, the locking ring is positioned opposite a circumferential groove in the wall of the wellhead bore. The cam ring is rotated by a running tool and is caused to move against the locking ring moving it into engagement with the inner groove of the wellhead assembly. Removal of the hanger is only possible once the cam ring is rotated in the opposite direction, moving it out of engagement with the locking ring, in turn allowing the locking ring to release from the internal groove of the wellhead assembly. In this way, the holddown apparatus retains the hanger locked within the wellhead assembly. Due to imprecision in the forming of the locking ring and the inner groove of the wellhead assembly, relative movement between the holddown and hanger apparatus and the wellhead can occur. As discussed above, this leads to a rapid wearing of the seals acting between the hanger assembly and the inner wall of the wellhead.
U.S. Pat. No. 5,307,879 discloses a casing or tubing hanger assembly having a lockdown apparatus for locking the seal of the hanger against the inner wall of wellhead assembly into which the hanger is installed. The lockdown apparatus includes a split lock ring which is movable radially into and out of engagement with a circumferential groove in the wall of the inner bore of the wellhead assembly. A wedge ring is provided for vertical movement. Downwards movement of the wedge ring brings it into contact with the segments of the lock ring, which action moves the lock ring into engagement with the inner groove of the wellhead. Once the lock ring is engaged, the hanger assembly is retained in the wellhead bore. Further downward movement of the wedge ring is provided, which acts to set the seal, once the lock ring is in full engagement with the groove of the wellhead. This further movement is provided by allowing the wedge, ring to partially deform under the action of continued applied downwards force once the lock ring is fully engaged. Again, even though the lock ring of the assembly of U.S. Pat. No. 5,307,879 may fully engage in the internal groove of the wellhead, vertical movement of the lockdown apparatus and the hanger assembly is possible, leading to the rapid wear of the seals discussed above.
It can be seen, therefore, that there is a need for an improved manner of securing and retaining assemblies, such as tubing or casing hangers, within the bore of a wellhead, such that relative vertical movement of the assembly with respect to the wellhead assembly is prevented.
U.S. Pat. No. 5,145,006 discloses a tubing hanger and running tool with preloaded lockdown. The hanger lands on a seat within a subsea wellhead housing. A locking means is provided on the hanger to engage an interior recess in the wellhead housing. A preloading means includes an extendible ring cooperating with the locking means to provide a preload to the hanger and prevent movement of the hanger within the housing.
A tubing hanger has a very specific use within a wellhead assembly, that is the support and retention of tubing extending within the well. A tubing hanger is a complex item of equipment. In keeping with this complexity, it has been found that it can be difficult to provide the tubing hanger itself with the preload means, as well as retain the other features of the hanger necessary for it to function properly. Accordingly, an improved device is required that may be applied within a wellhead, for example to secure a tubing hanger, as well as a range of other apparatus within a wellhead housing, in such a way that relative movement between the device, together with any apparatus it is retaining, and the wellhead housing is prevented.
According to a first aspect of the present invention, there is provided a retaining apparatus for use in a wellhead, the wellhead having a central bore and first and second retaining means within the bore of the wellhead, the apparatus comprising:
a housing for location within the bore of the wellhead;
a first engagement assembly for engaging with the first retaining means in the bore of the wellhead to thereby retain the housing within the bore of the wellhead;
a second engagement assembly for engaging with the second retaining means in the bore of the wellhead;
a preload assembly operable when the housing is located within the bore of the wellhead to apply a preload to the housing between the first and second retaining means.
The apparatus of the present invention is retained in the bore of the wellhead assembly in a manner such as to prevent longitudinal movement of the apparatus and any other wellhead components retained by the apparatus within the bore. The application of the preload to the apparatus after its initial installation serves to lock the apparatus in place and prevent the aforementioned movement. By preventing such movement, the problems and shortcomings discussed above are overcome. In particular, the rapid wear of the seals in place between the apparatus and the inner wall of the wellhead assembly is prevented, in turn preventing premature failure of the seals. In addition, it has been found that components of the wellhead assembly introduced into the central bore through horizontal ports, for example electrical and hydraulic control lines, suffer from rapid wear and failure due to vertical movement of components, such as casing or tubing hangers, in the central bore. Again, by eliminating the vertical movement of the components within the wellhead bore, the poor performance and failure of equipment introduced horizontally into the wellhead bore can be prevented.
Suitable assemblies for use as the first retaining means within the wellhead bore are known in the art and include such arrangements as one or more circumferential grooves in the inner wall of the wellhead assembly. The first retaining means within the bore of the wellhead preferably comprises an internal shoulder within the bore. In this case, the first engagement assembly is most conveniently a shoulder provided on the housing for seating directly or indirectly on the wellhead shoulder. By direct seating is meant that the shoulder of the housing is in direct contact with the internal shoulder of the wellhead assembly. The shoulder of the housing may seat indirectly on the internal wellhead shoulder when one or more wellhead components, such as one or more casing hangers, are installed between the retaining apparatus and the internal shoulder. In such a case, the shoulder of the housing seats against the or the uppermost intermediate component. In such an arrangement however, the internal shoulder of the wellhead still acts to retain the housing of the apparatus.
As the second retaining means the wellhead may comprise any suitable assembly known in the art. Preferably, the second retaining means comprises a circumferential groove within the bore of the wellhead. In such a case, the second engagement assembly is of a form to engage with the circumferential groove, most conveniently comprising a locking ring engageable with the circumferential groove. To operate the second engagement assembly, the apparatus may comprise an actuator movable between a first position in which the actuator is disengaged from the locking ring to a second position in which the actuator is engaged with the locking ring, movement of the actuator from the first position to the second position when the apparatus is positioned within the wellhead engaging the locking ring with the circumferential groove. Preferably, the actuator is a collar moveable longitudinally with respect to the housing. In one embodiment, the collar comprises a tapered surface, movement of the collar from the second position to the first position bringing the tapered surface into contact with the second engagement assembly and thereafter urge the second engagement assembly into engagement with the circumferential groove.
In the apparatus, the first and second engagement assemblies are spaced apart. The preload arises from the action of the preload assembly urging the first and second engagement assemblies into a position of greater separation, thus generating a force or preload between the two. This serves to secure the apparatus within the bore of the wellhead assembly and prevent its movement. The preload assembly preferably comprises a loading member movable so as to be brought to bear against the second engagement assembly once the second engagement assembly is engaged with the first retaining means, to urge the apparatus towards the first retaining means within the wellhead and, thereby, provide the preload. In one embodiment, the loading member is a loading ring movable longitudinally with respect to the housing. In one convenient arrangement, a threaded connection secures the loading ring to the housing, rotation of the loading ring with respect to the housing causing the loading ring to move longitudinally with respect to the housing.
As already noted, generally, casing or tubing hangers tend to be complex components, causing difficulty in providing means in the hanger to apply the preload. Accordingly, the apparatus of the present invention is provided as a separate wellhead component, in order to apply the preload to other wellhead components, such as casing or tubing hangers, indirectly. It has been found that the apparatus performs very well as an internal tree cap, in particular an internal tree cap arranged to be landed and secured within a spool tree in the wellhead assembly. As noted above, the internal tree cap may be arranged to be landed directly on a retaining means, such as an internal shoulder, within the bore of the spool tree. In this arrangement, the internal tree cap will apply a preload to itself to be retained within the wellhead housing. An example of when such an installation is of use is when horizontal control lines are provided in the wellhead assembly to extend through the housing and into the internal tree cap itself.
Alternatively, the internal tree cap may be arranged to land on a casing or tubing hanger, which in turn is landed directly on a retaining means within the wellhead spool tree.
In a further aspect, the present invention provides a method for retaining an apparatus within the central bore of a wellhead assembly, the method comprising:
providing a first retaining means within the bore of the wellhead assembly;
providing a second retaining means within the bore of the wellhead assembly;
providing the apparatus with an engagement assembly for engaging with the second retaining means within the bore of the wellhead assembly;
positioning the apparatus within the bore of the wellhead assembly so as to be retained by the first retaining means;
engaging the engagement assembly with the second retaining means;
applying a preload to the apparatus against the second retaining means to urge the apparatus toward the first retaining means.
As discussed above, the first retaining means is preferably an internal shoulder formed within the central bore of the wellhead assembly. The apparatus may be landed directly on the internal shoulder within the central bore of the wellhead. Alternatively, the method may comprise first landing a wellhead component, such as a casing or tubing hanger, within the central bore of the wellhead assembly so as to be retained by the first retaining means, the apparatus thereafter being landed directly on the wellhead component.
In one preferred embodiment, the second retaining means within the bore of the wellhead assembly comprises a circumferential groove, the apparatus being provided with an engagement ring movable radially into and out of engagement with the circumferential groove once the apparatus is positioned within the central bore of the wellhead assembly. Preferably, the method further comprises providing an actuator movable longitudinally within the central bore of the wellhead assembly, movement of the actuator in a first direction causing the engagement ring to move into engagement with the groove and movement of the actuator in a second direction causing the engagement ring to move out of engagement with the groove. To install the apparatus in the wellhead bore, the method comprises moving the actuator in the first direction to cause the engagement ring to engage with the groove.
To apply the preload, the apparatus is preferably provided with a loading assembly, the loading assembly being movable longitudinally within the bore of the wellhead assembly, with the apparatus positioned within the bore of the wellhead assembly and the engagement assembly engaged with the second retaining means, movement of the loading assembly in a first direction causing the loading assembly to bear against the second retaining means and urge the apparatus against the first retaining means, thereby applying the preload to the apparatus. To fully install the apparatus in the wellhead bore, the method includes moving the loading assembly in the first direction to bear against the second retaining means and thus apply the desired amount of preload.
In a preferred embodiment, in which the apparatus comprises a housing, movement of the loading assembly in the first direction is obtained by rotation of the loading assembly with respect to the housing, the housing and the loading assembly being connected, for example, by means of a threaded connection allowing the necessary relative rotation.
In a further aspect, the present invention provides an internal tree cap for installation in the central bore of a wellhead assembly, the wellhead assembly comprising first and second retaining means within the bore of the wellhead, the internal tree cap comprising:
a cap housing for location within the bore of the wellhead;
a first engagement assembly for engaging with the first retaining means in the bore of the wellhead to thereby retain the cap housing within the bore of the wellhead;
a second engagement assembly for engaging with the second retaining means in the bore of the wellhead;
a preload assembly operable when the cap housing is located within the bore of the wellhead, the preload assembly when operated bearing against the second retaining means to urge the cap housing towards the first retaining means, the preload assembly thereby applying a preload to the cap housing between the first and second retaining means.
In still a further aspect, the present invention provides a wellhead assembly comprising:
first and second retaining means within the bore of the wellhead;
an internal tree cap retained in the bore of the wellhead, the internal tree cap comprising:
a cap housing for location within the bore of the wellhead;
a first engagement assembly for engaging with the first retaining means in the bore of the wellhead to thereby retain the cap housing within the bore of the wellhead;
a second engagement assembly for engaging with the second retaining means in the bore of the wellhead;
a preload assembly operable when the cap housing is located within the bore of the wellhead, the preload assembly when operated bearing against the second retaining means to urge the cap housing towards the first retaining means, the preload assembly thereby applying a preload to the cap housing between the first and second retaining means.
The wellhead assembly may further comprise a spool tree, the internal tree cap being retained in the central bore of the spool tree. In many cases, the wellhead assembly will comprise a tubing hanger, the tubing hanger being retained in the bore of the wellhead assembly by the first retaining means, the first engagement assembly of the internal tree cap engaging with the tubing hanger to retain the internal tree cap within the bore of the wellhead assembly.